184 THE EVOLUTION OF PLANTS 



in a species of Bothrodendron, a genus in some 

 respects intermediate between Lepidodendron and 

 Sigillaria, the sporangium was attached by a 

 small base and the sporophyll was correspond- 

 ingly short. The long base of the sporangium in 

 Lepidodendrea3 finds a close parallel in Isoetes 

 alone among living Lycopods. 



The sporangia of the fossil genus often contain 

 bands of sterile tissue, running up from the sporo- 

 phyll, and perhaps helping hi the nutrition of 

 the spores. It will be remembered that the same 

 thing occurs in the sporangia of Isoetes. 



In some cases the two kinds of sporangia occur 

 in the same cone, the microsporangia above and 

 the megasporangia below, quite like an ordinary 

 SelagineUa. In other cases, however, it appears 

 that each cone bore one kind of sporangium only. 

 It is impossible to prove that all the species were 

 heterosporous, for sometimes only the small 

 spores have been observed, but the cones agree 

 so closely in structure, so far as the true Lepido- 

 dendrese are concerned, that there is little room 

 for doubt. The microspores were enormously 

 numerous; in the larger sporangia there were a 

 considerable number of megaspores, but in other 

 cases, as in the Bothrodendron cone already men- 

 tioned, there were only four. The difference in 

 size between the two kinds of spore was very 

 great, often 50 to 1 in diameter, which is equiv- 



